To provide a restroom pit latrines of a ship that can make water drain automatically and can be cleaned by itself.

The tank body of a holding tank 1 includes: an opening 2 for sea water inflow; an opening 3 for discharging both human excreta within the tank and sea water; a compressed air blowing port 6 for feeding air into the tank; a sea water forced discharging port 7 for forcedly discharging sea water remained in the tank; and an air exit port 5. Although human excreta and domestic sewage stored in the tank are discharged out during cruising at oceanic region, at this time, the valves at the opening 2 for sea water inflow and the opening 3 for sea water outflow are opened and the sea water forced discharging part 7 and the compressed air blowing port 6 are closed. After this operation, the sea water forced discharging valve 7 arranged at the holding tank 1 is opened and air is fed through the compressed air blowing port 6. The fed air is started to accumulate from the upper part of the holding tank to push the sea water remained in the tank in a downward direction.